I want to capture sntp packets from system(windows 8) running as both server and client(using a tool). my question is how can i capture ntp packets from this tool using wireshark in other system(windows xp) while server client tool(both as server and client) is synchronizing time in unicast mode.

ya like single system(system 1)works as both server and client and one more system(system 2) to capture packets(from system 1) using wireshark

Can't work and won't work. System 1 is communicating with system 1 (i.e., with itself) over an internal connection within the operating system software, NOT over an Ethernet or Wi-Fi network or anything else to which system 2 has any access whatsoever, and, unfortunately, unlike many UN*Xes, that internal network can't be sniffed using the same mechanism that is used to sniff external networks, so Wireshark can't see the traffic even on system 1 itself.

If the same computer is being used as both a client and a server, and that computer is using itself as a server, so that all the messages are sent from the computer to itself, you cannot capture them by using some other system; those packets are NOT transmitted on ANY network, much less a network that some other computer can sniff on.

On Windows, about all you can do in that case is run RawCap and have it write out to a file, and then read the file in Wireshark (or TShark or tcpdump/WinDump or...).